In the field of wireless communication systems, such as telecommunication systems, it is often desired to manage resources as efficient as possible. The resource may be a radio resource, such as a Resource Block (RB) or the like. In this manner, high data rates may be obtained. One aspect, affecting how to manage resources, concerns a user equipment's relation to its neighboring cells, which e.g. may be determined by performing measurements. Consider a scenario, in which the user equipment (UE) is served by a serving cell at a first frequency. In order for the user equipment to perform measurements on cells at frequencies being different from the first frequency, it may be necessary for the user equipment to interrupt communication with the serving cell. A reason for that the interruption is required is that the receiver and transmitter of the user equipment may only be configured for operation at one frequency, i.e. frequency range, at the time. This interruption is often referred to as a measurement gap, since the user equipment may perform measurements on the others cells during the interruption of communication with the serving cell, which sometimes is referred to as a Primary Cell (PCell). In case of carrier aggregation, (CA) where the user equipment is configured with a PCell and at least one secondary cell (SCell), the interruption of communication may occur on both the PCell and the SCell(s).
Measurements in Long Term Evolution (LTE) Using Gaps
As a general rule the user equipment performs inter-frequency and inter-Radio Access Technology (RAT) measurements in measurement gaps unless it is capable of performing them without gaps. To enable inter-frequency and inter-RAT measurements for the UE requiring gaps, the network has to configure the measurement gaps. Two periodic measurement gap patterns both with a measurement gap length of 6 ms are defined for LTE:                Measurement gap pattern #0 with repetition period 40 ms        Measurement gap pattern #1 with repetition period 80 ms        
The measurements performed by the UE are then reported to the network, which may use them for various tasks.
The following measurements are specified or can be performed by LTE UE which are done in measurement gaps:                Inter-frequency cell detection or cell identification        Inter-frequency Reference Signal Received Power (RSRP) measurement        Inter-frequency Reference Signal Received Quality (RSRQ) measurement                    Inter-frequency reference signal time difference (RSTD)            Inter-RAT cell identification; example of RATs are Global System for Mobile communication (GSM)/GSM Enhanced Data rates for GSM Evolution (EDGE) Radio Access Network (GERAN), WCDMA, Universal Terrestrial Radio Access (UTRA) Time Division Duplex (TDD), CDMA2000 etc.            Inter-RAT measurements e.g. Common Pilot Channel (CPICH) (Received Signal Code Power) RSCP, CPICH Ec/No, GSM carrier Received Signal Strength Indicator (RSSI) etc.                        
The measurement gaps are used in all duplex modes of operation e.g. Frequency Division Duplex (FDD), TDD and half-duplex (HD)-FDD (aka HD for simplicity). In half duplex (HD) or more specifically half duplex FDD (HD-FDD) the uplink and downlink transmissions take place on different paired carrier frequencies but not simultaneously in time in the same cell. This means the uplink and downlink transmissions take place in different time resources e.g. symbols, time slots, subframes or frames. In other words uplink and downlink subframes do not overlap in time. The number and location of subframes used for downlink (DL), uplink (UL) or unused subframes can vary on the basis of frame or multiple of frames.
Alignment of E-UTRA TDD Measurement Gaps with Particular Subframe Offsets
The frame structure used for E-UTRAN TDD is illustrated in FIG. 1. FIG. 1 shows frame structure type 2, used for TDD, with a 5 ms switching point. The uplink-downlink configurations used therewith are listed in Table 1 below.
TABLE 1Uplink-downlink configurations.Uplink-Downlink-downlinkto-UplinkCon-Switch-pointSubframe numberfigurationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
It can be noted that for Uplink-downlink configuration 0, measurement gaps with offsets 3 and 8 subframes relative to the frame border will be squeezed in between two uplink subframes, see FIG. 2. Moreover it can be noted that for Uplink-downlink configurations 0, 1 and 6, measurement gaps with offsets 2 and 7 subframes will be squeezed in between a special subframe, consisting of a downlink part, a guard period and an uplink part, and an uplink subframe, see FIG. 3.
Timing for Measurement Gaps
One of the assumptions when defining existing UE behavior for measurement gaps was that the measurement gap was to be defined with respect to the downlink timing, i.e., it was to be aligned with DL subframes. Moreover, it was assumed that transmission that would be overlapping the measurement gap were to be dropped. The LTE specification Third Generation Partnership Project (3GPP) Technical Specification (TS) 36.133 V10.14.0 defines the following UE behavior:
In the uplink subframe occurring immediately after the measurement gap,                the Evolved-UTRA network (E-UTRAN) FDD UE shall not transmit any data        the E-UTRAN TDD UE shall not transmit any data if the subframe occurring immediately before the measurement gap is a downlink subframe.        
The second bullet covers LTE TDD but does not cover the case when the measurement gap is positioned between two uplink subframes, or between a special subframe and an uplink subframe. This might be justified if considering only the autonomous change of timing; and this since the UE only is allowed to autonomously change the transmit timing by at most 17.5×Ts (0.57 μs) per 200 ms provided that it is not the first transmission after Discontinuous Reception (DRX). The relative position of the gap would differ since it is defined from UL timing instead of DL timing, but the length would be 6 ms, as required.
In a practical implementation, at some point in time the UE has to plan for switching a radio receiver thereof from intra-frequency to inter-frequency, and later back again. Additionally, the UE may need to plan for when to carry out automatic gain control, need access to common reference signals, when to start recording IQ samples for offline processing, and/or configure hardware accelerators for online processing, and/or configure software for control and processing. Suppose that this planning is done say less than 200 ms in advance—then the autonomous change of timing would potentially result in that the gap would move at most ±0.6 μs in addition for measurement gaps that are positioned between uplink activities. This could be handled by removing 0.6 μs from the beginning and the end of the measurement gap, as a margin for change in position. The impact would be negligible.
Timing Advance
In order to preserve the orthogonality in the uplink (UL) the UL transmissions from multiple user equipments (UEs) in LTE need to be time aligned at a receiver, such as a base station, the eNode B or the like. This means the transmit timing of the UEs, which are under the control of the same eNode B, should be adjusted to ensure that their received signals arrive at the eNode B receiver at the same time or more specifically their received signals should arrive well within the cyclic prefix (CP). Normal CP length is about 4.7 μs. This ensures that the eNode B receiver is able to use the same resources, i.e. same Discrete Fourier Transform, DFT, or Fast Fourier Transform, FFT resource, to receive and process the signals from multiple UEs.
The UL timing advance (TA) is maintained by the eNode B through timing advance commands, aka timing alignment commands, sent to the UE based on measurements on UL transmissions from that UE. For example the eNode B measures two way propagation delay or round trip time for each UE to determine the value of the TA required for that UE.
For a timing advance command received on subframe n, the corresponding adjustment of the uplink transmission timing shall by applied by the UE from the beginning of subframe n+6.
The timing advance command indicates the change of the uplink timing relative to the current uplink timing of the UE transmission as multiples of 16 Ts, where Ts=32.5 ns and is called basic time unit in LTE.
In case of random access response, an 11-bit timing advance command, TA, for a Timing Advance Group (TAG) indicates NTA values by index values of TA=0, 1, 2, . . . , 1282, where an amount of the time alignment for the TAG is given by NTA=TA×16. NTA is defined above in section “Alignment of E-UTRA TDD measurement gaps with particular subframe offsets”.
In other cases, a 6-bit timing advance command, TA, for a TAG indicates adjustment of the current NTA value, NTA,old, to the new NTA value, NTA,new, by index values of TA=0, 1, 2, . . . , 63, where NTA,new=NTA,old+(TA−31)×16. Here, adjustment of NTA value by a positive or a negative amount indicates advancing or delaying the uplink transmission timing for the TAG by a given amount respectively.
Timing advance updates are signaled by the evolved Node B (eNB) to the UE in MAC PDUs.
The discussion about timing for the measurement gaps is now resumed. When taking timing advance (TA) commands into account, it becomes somewhat more problematic. Change of timing has no impact on the measurement gaps that are covered by the text above since their positions are determined by the DL timing, but may have a big impact on the gaps whose positions are determined by the UL timing, i.e. those listed in the previous subsection. Although not very likely, the UE can receive one TA command every DL or special subframe to be applied 4 subframes later. Each such TA command may change the UL timing within the range −31×16 Ts to 32×16 Ts (about ±17 μs). If say assuming that the aforementioned planning is done 20 ms in advance, it would mean that the maximum timing change would be about ±180 μs for Uplink-downlink configuration 1. How much of this that actually can be applied depends on special subframe configuration, e.g. size of guard period, and aggregated timing advance at the time when the planning is carried out, e.g. when the 20 ms period begins since the aggregated timing advance is bounded. If handling the uncertainty in position of the measurement gap due to potential change of UL timing using the same approach as for the autonomous change of timing, the measurement gap will have to be reduced by, in worst case, about 0.36 ms. This is because the UE has to plan for the maximum of the aggregated TA change in either direction 20 ms in advance. This would leave too little time for the gap to be useful for cell search and measurements.
Considering the above analysis the minimum guaranteed measurement gap is analyzed for the following scenarios:                (a) FDD single component carrier, Rel.8 and onwards        (b) TDD single component carrier, Rel.8 and onwards        (c) FDD Carrier Aggregation (CA), Rel.10 and onwards        (d) TDD CA with same UL/DL allocation on both carriers, Rel.10 and onwards for single TAG, and Rel.11 and onwards for multiple TAGs. Gap is positioned between UL subframes.        (e) TDD CA with different UL/DL allocation on the carriers, Rel.11 and onwards. But there are no performance requirements in Rel-11. Gap on one carrier is positioned between UL subframes.        
Illustrations are provided in FIG. 4 and minimum guaranteed measurement gap length as well as mitigation to achieve at least 6 ms is provided in Table 2 below. FIG. 4 thus illustrates scenarios for which resulting measurement gap length is analyzed. Timing advance commands received during the 6 subframes before the gap will modify the length of the gap. Striped subframes are those where no serving cell transmission or reception is to be carried out under rules according to prior art.
TABLE 2Analysis of minimum guaranteed measurement gap both withand without received TA commands before the gap, andmitigation to guarantee a minimum gap length of 6 ms.Minimum gapMinimum gapwhen no TAwhen TAcommands areimmediatelyMitigation toreceivedbefore, to beguaranteeimmediatelyapplied duringminimum 6 msScenarioDescriptionbefore the gapgapgap(a)Single carrier6 ms6 msNothingFDD cell(No impact sinceneededfirst UL after gapis dropped)(b)Single carrier6 ms6-4 × 0.0167 =Drop UL afterTDD cell, UL/DL5.93 msgapconfiguration 0(4 TA commandsmay have to beapplied duringgap)(c)CA of FDD cells;6 − 0.030 = 5.975.97 msDrop DL aftersingle TAGms(No impact sincegap(UE shall handlefirst UL after gapDL timing offsetis dropped)of up to 30.26 usbetween PCelland SCell(s)(d)DL & UL CA of6 − 0.032 = 5.975.97-4 × 0.0167 =Drop UL afterTDD cells withms5.90 msgapsame(TA difference(4 TA commandsconfiguration,betweenmay have to beUL/DLmultiple TAGsapplied duringconfiguration 0shall be at mostgap, and hence32.47 us)shorten it)(e)DL & UL CA of6 − 0.032 = 5.975.97-4 × 0.0167 =Drop UL afterTDD cells withms5.90 msgapdifferent(TA difference(4 TA commandsconfiguration,between TAGsmay have to beUL/DLshall be at mostapplied duringconfigurations 032.47 us)gap, and henceand 5,shorten it)respectively
In some scenarios such as in large cell size and also when UL CA is used the UE will have to shorten the measurement gap e.g. from 6 ms to 5.90 ms. This is needed to ensure that the UE is able to communicate with the serving cell in the subframe after the gap. This shortening of the gap will however degrade the mobility performance, since UE after taking into account the frequency switching from serving to non-serving carrier and vice versa—total is 1 ms—is left with less than 5 ms, i.e. 5.90 ms reduced by 1 ms, for actual measurement. At least 5 ms is needed to ensure cell search which requires Primary Synchronization Signal/Secondary Synchronization Signal (PSS/SSS) in LTE and which are sent every 5 ms. A problem is hence that the time left for measurement is too short.
In some scenarios such as when no UL CA is used and when UE is close to the base station, then the network node may not have to send large TA commands to the UE. In this case the UE may not have to shorten the measurement gap and can operate with serving cell in the subframe after the gap. A problem is hence if the time left for measurement is increased due to the previous problem, the measurement time is unnecessarily long in this case.
Similarly in some scenario, where UE needs to be served with high data rate, then it is beneficial that UE operates, e.g. sends or receives data to/from its serving cell, in subframe immediately after the gap.